Among the several methods for initiating free-radical polymerizations are redox reactions using metal salt oxidizing agents and organic aldehyde or ketone reducing agents. However, the use of aldehydes and ketones in this fashion has been fairly limited in spite of their widespread availability and range of potential functional groups contained therein.
Additionally, of the wide range of metal salts available, copper (Cu) salts have found limited success as polymerization initiators in spite of their widespread availability and low cost. In fact, in some instances copper salts have been found to inhibit the reaction.
Many transition metal salts are capable of oxidizing aldehydes and ketones. The oxidation of ketones with the powerful oxidants dichromate ([Cr.sup.VI.sub.2 O.sub.7 ].sup.2-) and permanganate ([Mn.sup.VII O.sub.4 ].sup.1-) forms carboxylic acids by inner-sphere cleavage of the ketone carbon skeleton. Oxidation of ketones by Pb.sup.IV or Tl.sup.III acetates, however, leaves the carbon skeleton intact but results in rapid inner-sphere acetoxylation of the ketone. Neither of these outcomes produces a free radical that can be used to initiate vinyl polymerizations. Moreover, permanganate and dichromate can oxidize olefin bonds (such as those present on vinyl monomers) to 1,2-glycols.
It is known that the use of metal-organic redox systems based on powerful single electron transfer (SET) oxidants such as Co.sup.III, Ce.sup.IV, Mn.sup.III and V.sup.V (typically in acidic solutions) to initiate free radical polymerizations of styrene, methyl methacrylate, acrylates and acrylonitrile reveal the following undesirable phenomena:
(1) oxidative termination of polymerization by the metal ion is usually observed, especially with donor-type monomers (e.g., styrenes) that can stabilize the cationic charge produced by outer-sphere oxidation of their propagating free radicals; PA1 (2) Co.sup.III, Ce.sup.IV and Fe.sup.III ions have been shown to induce efficient conversion of various monomers to free radicals; and PA1 (3) oxidation of functional groups such as alcohols by these metal ions (even aromatic and aliphatic hydrocarbons such as biphenyl and cyclohexane by acidic Co.sup.III) limits the possible range of functional groups that can be present in the system.
Clearly, a large number of metal ion-based redox systems that can be used successfully in organic synthesis do not meet the demanding selectivity requirements imposed by homogeneous free radical polymerization systems.
In general, it can be easily seen, that there has existed a need for a redox initiating system that permits synthesis of controlled, predictable polymer end-group structures. Furthermore, it is highly desirable that the initiating redox system be relatively mild thereby permitting many other useful functional groups to be present and not be oxidized by the initiator.